This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-223479, filed Jul. 24, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a three-dimensional structure including a plurality of portions different in thickness, for example, an oscillator applied to an optical scanner.
2. Description of the Related Art
U.S. Pat. No. 6,188,504 discloses an optical scanner including a three-dimensional structure called as an oscillator, which is produced by selectively etching a semiconductor substrate. FIG. 18 shows the configuration of this optical scanner.
As shown in FIG. 18, the optical scanner comprises an oscillator 510 having a movable plate 512, a support frame 514 surrounding the movable plate 512, and a pair of elastic members 516 connecting the movable plate 512 and the support frame 514, a coil 522 extending along the periphery of the movable plate, a pair of wires 524 extending through the elastic members 516, respectively, a pair of feeding pads 526 formed on the support frame 514, and magnets 532 fixed to the support frame 514. The movable plate 512 has a reflecting surface 528 formed thereon to reflect a beam of light. The wires 524 have ends connected to the ends of the coil 522 and the other ends connected to the feeding pads 526. The elastic members 516 comprise an insulating elastic film such as polyimide resin. The insulating elastic film extends also over the movable plate 512 to function as an interlayer insulating film, which insulates the coil 522 from the wires 524.
In FIG. 18, when an AC voltage is applied to the pair of feeding pads 526, an AC current flows through the coil 522. Then, Lorentz force is generated owing to the interaction between the current flowing through the coil 522 and magnetic fields generated by the magnets 532. Thus, the movable plate 512 is subjected to a couple of forces exerted around an axis thereof passing through the interior of the elastic members 516. The directions of these forces depend on the direction of the current flowing through the coil 522. Since the AC current flows through the coil 522, the movable plate 512 oscillates around the axis passing through the interior of the elastic members 516. The oscillation of the movable plate 512 scans a beam of light reflected by the reflecting surface 528 of the movable plate 512.
Now, the process steps of manufacturing the oscillator 510 of this optical scanner 510 will be described with reference to FIGS. 19 to 22.
First, as shown in FIG. 19, a silicon nitride film 544 is formed on the major surfaces (top and bottom surfaces) of a silicon substrate 542. Then, the silicon nitride film on the bottom surface side is selectively etched to form a mask 548 used to form the movable plate and the support frame.
Then, as shown in FIG. 20, the coil 522, a polyimide film 552, the wires 524 and feeding pads 526, a polyimide film 554, and a polyimide etching mask 556 are sequentially formed on the silicon nitride film on the top surface side of the silicon substrate 542. An end of each of the wires 524 is electrically connected to a corresponding end of the coil 522 through a corresponding one of via holes formed in the polyimide film 552.
Subsequently, as shown in FIG. 21, with the top surface side of the silicon substrate 542 sealed, the silicon substrate 542 is selectively etched through the mask 548 from the bottom surface side with TMAH (Tetramethyl ammonium hydroxide) or the like, so that its portion that is not covered by the mask 548 is removed, to form the movable plate 512 and the support frame 514.
Furthermore, the polyimide films 552 and 554 are etched through the polyimide etching mask 556 to form the elastic members 516 (see FIG. 22). Finally, the polyimide etching mask 556 and the remaining silicon nitride film 544 and 548 are removed to obtain the oscillator 510 for an optical scanner, shown in FIG. 22.
In the oscillator 510 for an optical scanner, shown in FIG. 22, the thickness of each of the movable plate 512 and the support frame 514 is always the same as that of the silicon substrate 542, as is apparent from the method of manufacturing the oscillator. If the movable plate 512 is miniaturized, i.e. a dimension of the movable plate 512 such as the width W or length A thereof is reduced, the area of the coil decreases relatively to the volume of the movable plate 512. Consequently, the oscillator or scanner is less efficiently driven.
As the movable plate 512 is miniaturized, the dimension of the movable plate 512 such as the width W or length A thereof approaches the thickness of the silicon substrate 542. Accordingly, the movable plate 512 is shaped like a block as shown in FIG. 23. As a result, the position of the center of gravity 564 of the movable plate deviates from an oscillation axis 562. That is, as the movable plate 512 is miniaturized, the distance D from the oscillation axis 562 to the position of the center of gravity increases. This may cause unwanted vibration modes to be generated during driving.
Further, with the TMAH-based etching, which is most commonly applied to silicon etching, etching speed varies depending on the plane direction of silicon. Accordingly, the plane direction of silicon is selected according to the shape of a structure to be produced. A wafer with a plane direction (100) is used to form the movable plate described previously. In this case, the sides of the movable plate are tapered as shown in FIG. 23. The width WL or length LL of the top surface of the movable plate, in which the coil is formed, equals the width WS or length LS, respectively, of a mask 566 used to form the movable plate plus double the width (length) LT of the tapered portion. Therefore, the etching process with TMAH does not enable the formation of a movable plate in which the width WL or length LL of the top side is smaller than 2xc3x97LT.
The value of LT depends on the thickness of the silicon substrate. Accordingly, a thin silicon substrate 542 may be used to form a small movable plate. However, the thin silicon substrate 542 is not stiff and is thus not strong enough for handling during production. Further, the support frame 514 formed is as thick as the silicon substrate 542 used. Consequently, the produced oscillator 510 is not strong and is thus difficult to handle.
Therefore, with the method of manufacturing the oscillator 510 (three-dimensional structure) described previously, it is difficult to produce a very small movable plate (member).
It is a main object of the present invention to provide a method of manufacturing a three-dimensional structure, the method allowing portions different in thickness to be formed by a single etching process.
It is another object of the present invention to provide a method of manufacturing a three-dimensional structure, such as an oscillator, the method allowing a very small member, such as a movable plate, to be formed by wet etching.
It is yet another object of the present invention to provide a method of manufacturing an oscillator, the method allowing production of an oscillator that can be efficiently driven even with a small movable plate.
It is still another object of the present invention to provide a method of manufacturing an oscillator, which hardly generates unwanted vibration modes even with a small movable plate.
The present invention provides a method of manufacturing a three-dimensional structure (for example, an oscillator applied to an optical scanner or an acceleration sensor) having portions different in thickness, the method comprising: forming a laminated structure, which comprises at least two layers to be processed and at least one inner mask interposed between the layers, the layers and the inner mask being joined together, such that the laminated structure has top and bottom major surfaces; forming an outer mask on at least one of the major surfaces of the laminated structure; selectively etching the layers from one of the major surfaces of the laminated structure through the outer mask to expose the inner mask and then through the inner mask, so that the portions different in thickness are formed by one etching process.
Initially, portion that is not covered with the inner mask provided outside the laminated structure is selectively etched. Then, the inner mask provided inside the laminated structure is exposed. Subsequently, portion that is not covered with the inner mask is selectively etched. As a result, a three-dimensional structure having a thicker portion and a thinner portion is formed. The thickness of the thinner portion depends on the position of the inner mask in the laminated structure. Therefore, the thickness of the thinner portion of the three-dimensional structure can be independently and accurately controlled.
According to the present invention, a method of manufacturing a three-dimensional structure is provided, the method allowing portions different in thickness to be formed by a single etching process. Further, according to the present invention, an arbitrary portion of the structure can be formed to have an arbitrary thickness. Therefore, a very small member can be formed by wet etching.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.